1. Field of the Invention
The present invention relates to an LED drive circuit for driving an LED, and more particularly, to an LED drive circuit formed of a constant current circuit using a switching regulator IC.
2. Description of the Related Art
LEDs are used in various electronic devices. There are various drive circuits as an LED drive circuit, and an LED drive circuit formed of a constant current circuit using a switching regulator IC is used as well.
FIG. 7 is a circuit diagram of an LED drive circuit using a conventional step-down switching regulator IC. An LED drive circuit 1 using the conventional step-down switching regulator IC includes a step-down switching regulator IC 2A, a resistor circuit 3 for detecting a current of an LED 10 connected to an output terminal VOUT, and a switching transistor 4, a coil 5, a diode 6, and capacitors 7 and 8 which are external elements of the switching regulator IC 2A. In a case of this circuit, a configuration in which the current of the LED 10 is detected on a terminal VRET side is taken as an example.
FIG. 8 is a circuit diagram of an LED drive circuit for stepping down a power supply voltage to drive an LED using a conventional step-up switching regulator IC. An LED drive circuit 1 using the conventional step-up switching regulator IC includes a step-up switching regulator IC 2B, a resistor circuit 3 for detecting a current of an LED 10 connected to an output terminal VOUT, and a switching transistor 4, a coil 5, a diode 6, and capacitors 7 and 8 which are external elements of the switching regulator IC 2B. In a case of this circuit, a configuration in which the current of the LED 10 is detected on the output terminal VOUT side is taken as an example.
The LED drive circuits 1 as described above cause the LED to emit light by the following operation.
For example, in the LED drive circuit 1 of FIG. 7, the switching regulator IC 2A controls on/off of the switching transistor 4, and causes a current generated in the coil 5 to flow from the output terminal VOUT to the LED 10. An output current of the output terminal VOUT generates a voltage by flowing through the resistor circuit 3. The voltage in proportion to the output current is input to an FB terminal of the switching regulator IC 2A. The switching regulator IC 2A controls on/off of the switching transistor 4 so that the voltage becomes constant. Accordingly, the LED 10 emits light with constant luminance.
The LED drive circuit 1 of FIG. 8 also causes the LED 10 to emit light with constant luminance by the similar operation.
In a market, there is such a demand that the LED drive circuit is driven by power supply obtained by directly rectifying a commercial AC power supply, for example, by power supply of DC 100 V which is converted from AC 100 V.
However, the above-mentioned LED drive circuits 1 each have a circuit configuration in which a power supply voltage is directly applied to a power supply terminal Vdd of the switching regulator IC. Accordingly, the LED 10 cannot be driven by a power supply voltage which is equal to or larger than a breakdown voltage of an IC in the conventional LED drive circuits 1. For example, in the case of a circuit driven by a power supply voltage of 100 V, a switching regulator IC having a breakdown voltage of 5 V cannot be used. In such a case, a switching regulator IC having high breakdown voltage is used or a power supply voltage is stepped down. However, there arises such a problem that the switching regulator IC is costly in the former case while power efficiency deteriorates in the latter case.